I. Field of the Invention
The present invention relates generally to power steering systems for vehicles, and more particularly to electrically powered steering systems which include an electric drive motor for providing a powered assist to the steering gear of the host vehicle.
II. Description of the Prior Art
A variety of electrically powered steering systems (known commonly as electric power steering systems or "EPS systems") have been proposed for providing a powered assist to the steering of a motor vehicle. Conventional rack-and-pinion steering systems include a primary pinion/rack gear mesh interface for coupling the steering wheel of the vehicle to the steering system. EPS systems include an electric drive motor having a rotating element which is additionally mechanically or hydraulically coupled to the rack of the steering gear. EPS systems are said to provide fuel efficiency enhancement amounting to between about 21/2 and 5 percent; this enhancement is usually on the lower end of this range for relatively larger vehicles. EPS systems are also said to incorporate software which is easily programmable to provide selected steering characteristics for any particular vehicular application. However, despite overall industry developmental commitments to date on the order of a billion dollars (U.S.), no EPS system is currently offered for sale in a mass produced automobile in the United States. There are a variety of reasons why EPS systems are not provided on automobiles in this country. One reason is that EPS systems are generally subject to an "auto-steer" problem, in which an unintended steering event is possible. Another reason is that EPS systems generally provide unsatisfactory tactile feedback (or "feel") during use; colloquially, EPS systems simply "feel funny" in operation. The art generally does not satisfactorily indicate the source or sources of these tactile feedback problems. Another drawback to the use of EPS systems is the difficulty (as yet unmet) of imposing speed reduction means between the motor and the steering linkage of the vehicle without also imposing either or both of mechanical over constraint and mechanical backlash between the motor and steering linkage.
It is believed herein that the auto-steer problem occurs because the drive motor of such systems is directly linked to the host vehicle's steering linkage and both the magnitude and the direction of steering boost are determined in an open-loop manner. Moreover, prior EPS systems appear to lack even minimal safety feedback information, such as full time independent verification that instant values of the actual assistive force have been properly generated. Errors in the proper generation of the assistive force are not sensed by the driver and remain uncorrected by the system. Other complaints about prior EPS systems include "motor cogging," lack of return ability and poor steering response to small input signals.
Several methods are known for coupling the drive motor of an EPS system to the steering linkage of the host vehicle. One method entails the use of a drive motor having a hollow rotor in which the rack shaft of a rack-and-pinion steering gear is concentrically disposed. The drive motor and rack shaft are connected by a ball screw and ball nut assembly, the ball screw being positioned on the rack in place of the conventional power cylinder, and the ball nut being engaged with the ball screw and supporting one end of the rotor. The other end of the rotor is supported by a thrust bearing. Other methods for coupling the drive motor to the steering linkage include coupling the drive motor to a second gear rack, via a gear train and a second pinion/rack gear mesh interface generally similar to the primary pinion/rack gear mesh interface utilized for coupling the steering wheel to the steering system, or coupling the drive motor directly to the steering shaft via a gear train. The ball screw/ball nut configuration is problematic because it couples the motor torque into the rack along with the desired axial thrust. This torque is quite sufficient to overcome the preload of the rack into the pinion so additional rotational constraint must be applied to the rack. This causes an over constraint in the gear mesh interface relationship between the pinion and the rack which results in stick-slip tactile characteristics felt at the steering wheel.
Similarly, adding a second pinion/rack gear mesh interface provides over constraint between either pinion/rack gear mesh with similar deleterious results. If the second pinion/rack gear mesh is loaded by a yoke mechanism there is additional Coulomb friction which effects return ability. Coupling the drive motor directly to the steering shaft via a gear train is limited to vehicles with very light steering loads because of wear limitations in the primary pinion/rack gear mesh. With either gear train, backlash becomes a tactile issue because it can be felt at the steering wheel. And if such a gear train were loaded sufficiently to eliminate the backlash, sufficient coulomb friction would be added as to eliminate any semblance of on-center feel.
It is obvious that all such electromechanical assemblies should comprise a clutch for decoupling the reflected inertia of the rotating elements of the electric drive motor from the steering system in the event of an otherwise orderly shutdown which would result from any system failure. Apparently however, most EPS systems under consideration today are being specified without a clutch for reasons of economy. This is unfortunate because the resulting increase in steering effort will certainly become a safety issue in the event of such system shutdowns.
Generally, other than in providing an obvious solution to the electric drive motor decoupling issue, such mechanical problems can be overcome in an EPS system by providing a fluid coupling between the electric drive motor and the rack of the steering gear in which the selective supply of pressurized fluid to the ports of a power cylinder is carried out by a reversibly driven fluid pump, rather than by a mechanical gear or leadscrew reduction means of the type described above. In this case, the ports of the fluid pump are connected to the ports of the power cylinder by first and second fluid lines. In such a system, bulk cavitation is precluded in either side of the power cylinder otherwise subject to an absolute pressure value below atmospheric pressure by replenishment fluid flowing through either of a pair of check valves disposed in a suction line connected to a reservoir and connected one each to the first and second fluid lines. Since systems configured in this manner advantageously eliminate the four-way control valves conventionally associated with EHPS systems, they should be considered to be true EPS systems.
In addition, many previously known power steering systems, including EPS systems, have significant tactile problems at very low frequencies, in particular, on the general order of 1 Hz. This includes system resonance which is apparently ignored within the art but can readily be recognized by a driver sensing an "over-center" type of instability wherein the driver must either anticipate lateral vehicle motion, or tightly grip the steering wheel in order to maintain precise control of vehicle tracking.
One attempt to address some of these problems is provided in U.S. Pat. No. 5,473,539 (Shimizu et al., Dec. 5, 1995). That patent discloses an electrically operated power steering apparatus in a motor vehicle having a steering system. The apparatus comprises a steering torque detector for detecting a manual steering torque applied to the steering system, an electric motor for generating an assistive torque to be transmitted as a steering assistive force to the steering system, and an actual assistive torque detector which detects an actual assistive steering torque which is actually transmitted from the electric motor to the steering system. The apparatus also comprises a controller which generates a target value for the assistive torque to be generated by the electric motor, and which generates a control signal based on the difference between the actual assistive steering torque detected by the actual assistive torque detector and the target value, the control signal then being used to energize the electric motor.
In a first embodiment, the patent discloses an actual assistive torque detector 22 coupling the nut 11a of a ball screw mechanism 11 to a rack shaft 7 of the steering system, the assistive torque of the electric motor 10 being applied to the rack shaft 7 through the ball screw mechanism 11. The patent indicates that the actual assistive torque detector 22 may be a pressure sensor comprising a resistance wire strain gage. In a second embodiment, the detector 22 is replaced with an actual assistive torque estimator which estimates an actual assistive torque from the voltage across the electric motor and the current through it. Because the detector 22 and the estimator provide quantitative information about the magnitude of the actual assistive torque, quantitative information which is necessary for the rest of the disclosed parts of the system to act in the manner described in the patent, the detector 22 and estimator do more than merely "detect" or respond to the presence or absence of an actual assistive torque; instead, they actually measure or estimate its value. A third embodiment in the patent attempts to give the driver of the vehicle a comfortable feel of steering action by providing a high- and/or a low pass filter in the actual assistive torque detector. The disclosed purpose of such filters is to reduce the purported noise from harmonics which are generated in the system upon differentiation for conversion from a rotational angular speed into a rotational angular acceleration, such that the estimator takes into account the inertial torque and the viscosity torque with respect to the motor torque within the system, based on the motor current and the rotational angular speed.
This patent appears to reflect a belief throughout the automotive industry that the issue of poor tactile feedback can and should be addressed by increasingly complicated software control schemes wherein the applied steering boost is made to model the input steering effort. Such efforts, however, have lead to enormous development expenditures without commensurate results; the art appears to provide no guidance as to actually solving the low frequency problems described above.
The related problem of steering shudder was addressed by the method and apparatus for enhancing stability in servo systems disclosed in U.S. Pat. No. 5,544,715 (E. H. Phillips, Aug. 13, 1996). The whole of that patent is expressly incorporated by reference herein. The patent discloses the use of series damping devices to form compliant couplings having series damping characteristics, used either for mounting hydro-mechanically driven actuators, or for coupling them to load elements which they position. The series damping absorbed sufficient energy to provide adequate gain and phase margins for the feedback characteristics of systems utilizing such actuators, so as to substantially prevent the occurrence of high frequency shudder. Of particular interest are the general steering system characteristics described in the specification of the '715 patent and depicted in the block diagram shown in FIG. 3 of that patent. The '715 patent discloses mechanical devices and methods for achieving servo control of the open-loop feedback characteristics present in general steering systems.
Many prior EPS systems appear to experience only marginally stable control and suffer a resultant amplification of external disturbances to them. While general techniques for achieving servo control in other systems are discussed in a variety of textbooks, the application of such techniques to EPS systems would require a knowledge (presently unpossessed in the art) of precisely where undesired resonances arise in EPS systems. More particularly, an introduction to servo control which can easily be understood by a novice in this field can be found in a "crib" text book by DiStefano, Stubberud, and Williams entitled Schaum's Outline of Theory and Problems of Feedback and Control Systems and published by the McGraw-Hill Book Company. As pointed out in that book, any servo system having a closed feedback loop can oscillate via self excitation at any frequency whereat unity gain in an opened feedback loop coincides with an odd multiple of 180.infin. phase shift of that opened feedback loop's phase angle. The prior EPS systems mentioned above appear to be characterized by the near confluence of these conditions, with the resulting marginally stable control and amplification of external disturbances mentioned above.
The block diagram shown in FIG. 3 of the '715 patent is both complex and complicated, and discloses several feedback paths inherent in a variety of steering systems. However, block diagrams like that shown in FIG. 3 of the '715 patent can be reduced via appropriate algebraic manipulation to substantially simpler diagrams like that shown in FIG. 4 of the '715 patent. By such algebraic manipulation, all of the forward gain factors can be considered as being comprised within a single forward gain block "G", while all of the feedback gain factors can be considered as being comprised within a single feedback gain block "H".
The closed loop gain ratio O/I of a system whose analysis is reduced in this manner can be determined by the formula: EQU O/I=G/(1+GH)
wherein O is a particular output value of the system, I is a particular input value for the system, G is the forward gain value and H is the feedback gain value. It should be readily apparent that the closed loop gain ratio O/I becomes unstable at any frequency or frequencies where the open loop gain GH attains a value of minus 1, that is, where the absolute value of GH has a value of 1 and its phase angle is equal to an odd multiple of 180 degrees; the denominator of the ratio rapidly approaches zero, so that the gain rapidly approaches infinity. Similarly, the closed loop gain ratio O/I is at best only marginally stable at any frequency or frequencies where the open loop gain GH attains a value which is close to (but not equal to) minus 1. Failure to counteract or otherwise address any particular resonance associated with an open loop gain GH value close to minus 1 would result in such a system having marginal stability. Unfortunately, one such shortcoming appears to be typical of prior EPS systems, since it appears that the art as a whole has failed to recognize or correctly analyze the source of the resonance which causes the low frequency stability control problems mentioned above.
It would be highly advantageous to provide methods and apparatus for substantially eliminating stability problems in EPS systems, and, in particular, for substantially eliminating the low frequency stability control problems mentioned above. It would also be highly advantageous to provide full time independent verification of instant values of steering boost in EPS systems in order to substantially eliminate concerns relating to auto-steer. It would be particularly advantageous for such a system to achieve a substantially linear control relationship between an applied steering torque input and a resulting steering force output, and thereby achieve an optimum tactile relationship between a vehicle, the vehicle driver and the steering system of the vehicle. It would be further advantageous for such a system to operate in a regenerative manner and enjoy all of the benefits of a regenerative system. And, it would be still further advantageous to provide means for decoupling the reflected inertia of the electric drive motor in the event of any system failure.